1. Field of the Invention
The present invention relates to alginate impression materials provided in the form of powder. More particularly, the present invention relates to alginate impression materials which are low dusting, are excellent storage stability and are capable of giving a gypsum model with high precision.
2. Description of the Prior Art
Generally, alginate impression materials for dental use which are provided in the form of powder are inexpensive and permit preparation of impressions of an oral cavity with appropriate precision and therefore they have heretofore been used widely. Upon use, a predetermined amount of a powdery alginate impression material for dental use (hereinafter referred to as "alginate impression material" for brevity) and water are placed in a small rubber bowl and mixed using a spatula to form a paste, which is then put on a tray for impression, and the tray is introduced into the oral cavity of a person and pressed onto his portion from which it is desired to copy an impression. After the paste has gelled to form an elastic material, it is taken out of the oral cavity, thus preparing an impression of the portion in the oral cavity. Then, a gypsum slurry is poured into the negative model of the impression and a gypsum model as a working model for preparing a prosthetic appliance is obtained. How the detail of the surface of the gypsum is reproduced determines adaptability of the prosthetic appliance prepared when it is put on in the oral cavity. The roughness of the surface of the gypsum model depends on the conditions of the interface between the alginate impression material and the material of the gypsum model.
Therefore, in practice, those materials which accelerate or do not inhibit gelation of alginate impression materials or congelation of gypsum must be chosen.
When mixed with water, alginate impression materials become a paste. Further, in order to form a homogeneous gel elastic body, the alginate impression material comprises fine powdery components and contains particularly fine powders of diatomaceous earth, silicic anhydride, talc, calcium carbonate, pearlite, etc., generally in an amount of 50 to 80% by weight.
Such conventional powdery alginate impression materials cause precipitation during storage and therefore the bulk density of the powder itself tends to change gradually with the lapse of time. For this reason, it is common practice that upon use, a container in which the alginate impression material is stored is shaken to agitate the content and cause the powders precipitated to regain the smallest possible bulk density, and thereafter a predetermined amount of the powder is gathered with precision using a spoon having a definite surface area exclusively used for this purpose. When a lid or cap of the container is removed after the shaking of the container, dust is scattered outside of the container airborne. Further, dust is also formed when a predetermined amount of the powder is mixed with water in a small rubber bowl due to the agitation by a spatula used. The dust formed not only is unpleasant to users but also causes environmental pollution and is harmful to health. These are disadvantages of the conventional alginate impression materials.
In order to solve the above problems attempts have been made as described in Japanese Patent Application Laid-open No. 57-501426 to coat fine powders of an alginate impression material with a coating agent which can be wetted with water easily and rapidly. Examples of the coating agent used in this process include a natural polymer dispersing agent such as xanthane rubber, sodium polyalginate, etc., a cellulose ester or ether such as hydroxylethylcellulose, carboxymethylcellulose, etc., a synthetic nonionic surface active agent derived from polyethylene glycol, polypropylene glycol, polyol, an alkanolamine, a glycerol ester. These substances contain in the molecule thereof a hydrophilic group such as --COOH, --OH, --NH.sub.2, --CH.sub.2, --CH.sub.2 O--, etc. and have an excellent wettability with water or a substitute liquid for water.
However, this process (Japanese Patent Application Laid-Open No. 57-501426) has disadvantages that water absorption of the alginate impression material is increased, resulting in that depolymerization if the alginate contained therein due to presence of water as well as reaction between alkaline components and acidic components is accelerated. Therefore, the quality of product is deteriorated rapidly and storage time or shelflife is shortened considerably.
Further, no reference is made in Japanese Patent Application Laid-open No. 57-501426 to precision of the surface of a gypsum model prepared from the impression material used. The coating agent described in this prior art inhibits or retards congelation of the gypsum and as a result a gypsum model prepared by pouring gypsum slurry into the negative mold of an impression has a rough surface, which leads to low precision.
A technology described in Japanese Patent Application No. 58-98021 filed by the same applicant aims at reducing dispersibility of dust and improving storage stability by coating the powder with a coating agent comprising a nonionic surface active agent and a hydrophobic liquid having a vapor pressure of not higher than 3.15 mmHg at 20.degree. C. selected from hydrophobic liquid hydrocarbons, fatty acids, alcohols, oils, silicone, etc. However, this technology is not intended to smooth the surface of a gypsum model and improve the precision thereof.
Nonionic surface active agents do not inhibit gelation of alginate impression materials or congelation of gypsum. However, they are ineffective for smoothing the surface of a gypsum model prepared using an impression made of an alginate impression material containing the nonionic surface active agents. This is believed to be due to the fact that the nonionic surface active agent is not dissolved to form ions but is loosely bound with a great number of molecules of water through hydrophilic groups such as --OH, --O--, etc. and therefore hydration reaction of gypsum and subsequent growth of crystals of gypsum dihydrate during congelation will proceed relatively slowly, resulting in that crystals of gypsum dihydrate tend to grow easily.